1. Field of the Invention
The invention relates to arrangements for characterizing and reducing multi-path interference (MPI) and/or optical return loss (ORL). More specifically, the invention relates to arrangements, especially portable arrangements, for characterizing and reducing MPI and/or ORL in optical transmission links having a single or multiple discrete reflection sources.
2. Related Art
In many optical fiber transmission links, not only is there continuous reflection from the fiber itself, but there are also reflections from discrete reflection sources (see FIG. 1A) such as connectors, mechanical splices or “bad” (reflective) fusion splices. See L. A. Reith, “Issue relating to the performance of optical connectors and splices,” Proc. SPIE, Vol. 1972, pp. 294-305, 1996. Reflections from these discrete reflection sources may degrade the performance of an optical communication system considerably through multi-path interference (MPI). See J. Boromage, and L. E. Nelson, “Relative impact of multi-path interference and amplified spontaneous emission noise on optical receiver performance,” OFC 2002, paper TuR3. See also U.S. Patent Application Publication No. 2004/0120641 (Machewirth et al.).
In the context of optical fibers having discrete reflection sources, optical return loss (ORL) may be defined as a ratio (usually expressed in dB) of optical power reflected back from an interface and the optical power arriving at the interface. In contrast, MPI may be defined as a relatively complex propagation phenomenon that results in optical signals reaching a receiving device by multiple paths, such as by reflection from the multiple discrete reflection sources. MPI's effects include constructive and destructive interference that potentially are extremely complex.
Because the MPI signal co-propagates with the original signal (see FIGS. 1B, 1C) and may also have the same spectral structure as the original signal, direct measurement of MPI in the field is much more difficult than measurement of ORL. As a result, commonly, only ORL has been measured in the field; the level of MPI is merely estimated, based on the ORL measurement.
According to Telecordia's standard, ORL less than −27 dB is thought to be low enough that its impact of reflection on the optical communication system is negligible. However, recent investigations have revealed that even if the ORL is smaller than −27 dB, MPI can be severe if there are strong discrete reflections in the optical transmission link. For example, as shown in FIG. 2, if there are two strong (−18 dB) discrete reflection sources located 2 km apart and near the middle of an 80 km optical transmission span, the level of MPI can exceed −37 dB. This high MPI level is experienced even in purely EDFA-amplified optical communication systems, which give minimal MPI compared to EDFA/Raman hybrid system and all-Raman systems (assuming the same optical transmission link parameters).
However, an MPI of less than −40 dB per 80 km span is usually required for new generation ultra-long haul (ULH) wavelength division multiplexing (WDM) systems. Thus, even for purely EDFA-amplified systems, the level of MPI can be excessive even when ORL is within acceptable limits.
FIGS. 3A and 3B illustrate the relationship between ORL and MPI for respective fiber links. FIG. 3A illustrates the ORL-MPI relationship without multiple discrete reflection sources, and FIG. 3B illustrates the ORL-MPI relationship with such sources, for a purely EDFA system. FIG. 3A shows a deterministic relationship between ORL and MPI for a fiber link without discrete reflection sources: when ORL is smaller than −27 dB, the level of MPI is lower than −45 dB. This MPI value is beneath the −40 dB MPI threshold mentioned above for 80 km spans.
However, in the scenario of special interest here, a fiber link with multiple discrete reflections, FIG. 3B shows that the correlation between ORL and MPI is very weak. Thus, significantly, a small ORL doesn't necessarily imply a small MPI. New generation ULH transport WDM systems (with transmission distances exceeding 1500 km) impose much more stringent MPI requirements on the fiber link than old transport systems (with typical transmission distances less than 600 km). Accordingly, there is an urgent need to be able to characterize and reduce MPI.
Furthermore, if the level of MPI exceeds an allowed maximum level, the reduction of MPI to an acceptable level must be achieved at minimal cost. Traditionally, a field technician has been sent to visit almost all the discrete reflection sources for repair or replacement. Clearly, this traditional approach is very time consuming and expensive. Accordingly, there is also an urgent need in the art to reduce MPI to acceptable levels in a cost-efficient manner.
To better characterize a fiber link's MPI performance, two approaches for direct measurement of the level of MPI have been proposed and demonstrated.
The first approach is called the electrical method. See Chris R. S. Fludger and Robert J. Mears, “Electrical measurement of multipath interference in distributed Raman amplifiers,” J. Lightwave Technology, Vol. 19, no. 4, pp. 536-545, 2001. For this method, a continuous wave (CW) optical signal is fed into the fiber link at a transmitter, and the optical signal is then converted into an electrical signal at a receiver. An electrical spectrum analyzer determines the MPI level by analyzing the electrical spectra.
The second approach of direct MPI measurement is called the optical method. S. A. E. Lewis, S. V. Chemikov and J. R. Taylor, “Characterization of double Rayleigh scatter noise in Raman amplifier,” IEEE Photonics Technology Lett., Vol. 12, pp. 528-530, 2000. In this method, an input optical signal is modulated by a very high extinction ratio (>50 dB) optical modulator. At a receiver, the MPI signal is extracted from the original optical signal through another synchronized optical modulator that also has a very high extinction ratio. While the electrical method may be field applicable, the optical method is more suitable for laboratory applications due to the need of synchronization between the transmitter and the receiver.
Neither the electrical method nor the optical method deal with the problem of turning a fiber link with unacceptably high MPI into a link with acceptable level of MPI, especially doing so at minimal cost.
Other approaches to locating faults in optical fibers include coupling a second fiber to a fiber to be examined, and measuring the optical signal reflected from a far end of the second fiber; see U.S. Patent Application Publication No. 2004/0208503 (Shieh). Also, U.S. Pat. No. 5,999,258 (Roberts) discloses an MPI measurement system that analyzes transmitted light rather than reflected light.
Still other artisans have provided approaches to reduce existing MPI. U.S. Patent Application Publication No. 2003/0235360 (Mozdy et al.) discloses a method of reducing MPI by providing an optical pump that pumps a dispersion compensating optical waveguide with light. U.S. Patent Application Publication No. 2003/0011876 (Fidric) discloses an amplifier system in which an optical amplifier between two amplifier stages is by-passed so as to avoid introduction of MPI. However, especially in long haul fibers, reducing MPI is not as effective as altogether removing the sources of the MPI in the first place.
Unfortunately, the foregoing approaches do not effectively solve the problem of analyzing a given transmission system to facilitate reduction of MPI to below a given threshold, especially at minimal cost. Thus, there is a need in the art for cost-effectively characterizing and reducing MPI in optical transmission links having multiple discrete reflection sources.